Medium Acid Sites Research Articles

Copper Foam-Supported CuxO@Fe2O3 Core–Shell Nanotubes: An Efficient Ozonation Catalyst for Degradation of Organic Pollutants

Copper foam (CF)-supported CuxO@Fe2O3 core–shell nanotubes (CF/CuxO@Fe2O3 NTs) were synthesized as a hybrid catalyst for heterogeneous catalytic ozonation (HCO). The hybrid catalyst exhibits high activity and excellent stability for the HCO of organic pollutants. The total organic carbon removal rates of methyl orange, ibuprofen, and nitrobenzene via HCO were 89, 82, and 88%, respectively, which are over 2 times higher than the corresponding values obtained without the catalyst. The hybrid catalyst also displays wide pH operation range, low metal leaching, easy separation from treated water, and excellent recyclability. We prove that large amounts of Lewis acid sites with medium acidity are created at the CuxO/Fe2O3 interface due to the synergic effect of CuxO and Fe2O3. These medium acid sites are the main active centers for the production of free hydroxyl radicals (•OH) and superoxide radicals (O2•– or HO2•) in neutral solution. Moreover, the 3D porous framework of the catalyst enables easy dispersion in and separation from water. This work develops a new strategy to design HCO catalysts by combining strong and weak Lewis acid oxides and also opens a new avenue for developing HCO catalysts on a 3D porous framework.

Efficient and Stable O-Methylation of Catechol with Dimethyl Carbonate over Aluminophosphate Catalysts

The O-methylation of catechol is an effective method for the industrial production of guaiacol used as an important chemical. However, the low catechol conversion and poor catalyst stability are the most critical issues that need to be addressed. Herein, the O-methylation of catechol with dimethyl carbonate was investigated over aluminophosphate (APO) catalysts, using a continuous-flow system to produce guaiacol. APO catalysts were synthesized with varying P/Al molar ratios and calcination temperatures to study their effects on catalytic performance for the reaction. The physico-chemical properties of the APO catalysts were thoroughly investigated using XRD, NH3-TPD, CO2-TPD, FTIR, and Py-FTIR. The P/Al molar ratio and catalyst calcination temperature significantly influenced the structure and texture, as well as the surface acid-base properties of APO. Both the medium acid and medium base sites were observed over APO catalysts, and the Lewis acid sites acted as the main active sites. The APO (P/Al = 0.7) exhibited the highest catalytic activity and excellent stability, due to the suitable medium acid-base pairs.

Catalytic behavior of a ZnO/TiO2 composite in the synthesis of polycarbonate diol.

ZnO/TiO2 catalysts with different ZnO contents have been prepared through equal volume impregnation method, characterized by XRD, SEM, Py-IR, ICP, XPS, NH3-TPD and N2 adsorption/desorption, and evaluated in the synthesis of polycarbonate diol (PCDL) through transesterification. The results showed that titanium zinc oxide formed in these catalysts, and the content of acidic sites varied with the ZnO content, and ZnO/TiO2 (10%) has the highest acid amount. The ZnO/TiO2 (20%) with medium acidic sites showed the highest catalytic activity. The synthesis process of polycarbonate glycol was also optimized. Under the optimal reaction conditions, the yield of PCDL was 72.5%, and the M n reached 4829 g mol-1 with a PDI of 1.6.

Cooperative catalytic effects between the penta-coordinated Al and Al2O3 in Al2O3-AlPO4 for aldol condensation of methyl acetate with formaldehyde to methyl acrylate

The bare amorphous Al 2 O 3 -AlPO 4 and Cs/Al 2 O 3 -AlPO 4 catalysts were developed for the aldol condensation of methyl acetate with formaldehyde to methyl acrylate. The structure and property of catalyst were characterized by XRD, XPS, BET, Pyridine-IR, FT-IR, 27 Al-MASNMR, NH 3 -/CO 2 -TPD and SEM. The correlation between structural features and acid-base properties was established, and the loading effect of the cesium species was investigated. Due to cooperative catalytic effects between the penta-coordinated Al and Al 2 O 3 , the weak-Ⅱ acid and medium acid site densities and the product selectivity were improved. While the basic site densities of these catalysts were almost in proportion to the conversion of methyl acetate. The loaded Cs could form new basic sites and change the distribution of acid sites which further enhance the catalytic performance. As a result, the 10Cs/8AlP was proved to be an optimal catalyst with the yield and selectivity of 21.2% and 85% for methyl acrylate respectively. During the reaction, a deactivation behavior was observed on 10Cs/8AlP catalyst due to the carbon deposition, however, it could be regenerated by thermal treatment in the air atmosphere at 400 °C.

Enhanced Performance and Stability of a Trimetallic CuZnY/SiBEA Catalyst in Ethanol to Butadiene Reaction by Introducing Copper to Optimize Acid/Base Ratio

Bioethanol to butadiene is currently the most promising non-oil-based butadiene production route. Here, copper is introduced into the conventional bimetallic zeolite catalyst to partially substitute for zinc; the isolated tetracoordinated Cu(II) species are formed, with weak and strong basic sites transformed into medium acid sites in trimetallic CuZnY/SiBEA catalyst. A partial substitution of zinc by copper increases the dispersion of metal, reduces the formation of ZnO clusters, decreases the pore blockage, and enhances the total pore volume of catalyst. The Cu1Zn2Y5/SiBEA catalyst with an appropriate 0.33 Cu/(Cu + Zn) mass ratio, a highest medium acid sites/(weak + strong) basic sites value of 6.17, and largest total pore volume of 0.251 cm3/g in all samples presents excellent catalytic performance in the ethanol to butadiene reaction: 99.01% ethanol conversion and 73.36% butadiene selectivity, higher than most reported ethanol to butadiene catalysts. The isolated tetracoordinated Cu(II) structure is stable, which is beneficial to the stability of trimetallic catalyst; when the reaction time is 60 h, the butadiene selectivity is 45.95%, 14% higher than corresponding bimetallic catalyst. The butadiene productivity of Cu1Zn2Y5/SiBEA catalyst reaches up to 1.68 gBD·gcat−1·h−1 at WHSV = 6 h−1 and time-on-stream = 8 h. Increasing reaction temperature could linearly increase the ethanol conversion, while the butadiene selectivity increases first and then decreases, the suitable temperature is 375 ℃ for the highest butadiene yield.

Influence of nickel loading on the hydroisomerization of n-dodecane with nickel-tungsten oxide-alumina supported catalysts

Heterogeneous catalysts based on alumina-supported tungsten oxides (15 wt% W) with different loadings of nickel (0.5, 1, 2, 4, 6, 8 and 10 wt% Ni) were selected to study the influence of Ni loading on the hydroisomerization of n-dodecane. The catalysts were prepared by applying the wetness impregnation method on the supports to introduce W and Ni. The characterization techniques applied for determining physicochemical properties of the catalysts were N2 adsorption-desorption at 77 K (textural properties), X-ray diffraction (structure and crystalline phases), H2-TPR (redox properties), FTIR, NH3-TPD (acid sites analyses) and XPS (chemical surface analysis). The catalytic properties of such catalysts were found to be crucial in the n-dodecane conversion. The NH3-TPD profiles indicate that the medium acid sites are the main sites responsible for the reaction performance. The formation of bulky crystal structures of nickel species in the high nickel loading catalyst (10 wt% Ni) was confirmed by XRD and XPS results, resulting in the largest cracking activity. The conversion of n-dodecane and selectivity to i-C12+branched C12 tend to increase with Ni loading until the catalyst contains 6 wt% Ni (28% n-C12 conversion and 94% of branched C12 selectivity). The lower selectivity at high nickel loading is due to metal-based cracking reaction. An optimum balance between metal and acid centers is needed to achieve a compromise between conversion and selectivity, avoiding or minimizing cracking reactions.

Improving the coking resistance of Ni/Al2O3 modified by promoter (Ce, Zr, or Mg) under ethanol steam reforming: Perspective from growth sites of coking

A series of nickel catalysts supported on Al2O3 that modified with Ce-, Zr-, and Mg-promoter were prepared via a sol-gel method and a subsequent impregnation method for the ethanol steam reforming reaction (ESR). The effect of promoters on the carbon deposition was focused. Compared with Ni/Al2O3 catalyst, the introduction of different promoters decreased the selectivity for CH2CH2 to different extent, especially, Ce-, Mg- containing catalysts completely suppressed the production of CH2CH2 for 30 h tested at 500 °C. And the medium and strong acid sites were confirmed as the major sites for CH2CH2 production. Encapsulation carbon derived from ethylene takes the primary responsibility for carbon deposition, and the formation of metal carbide is the other reason for causing carbon deposition. Among the tested catalysts, Ni/Mg–Al2O3 exhibited a better performance for anti-carbon, which was attributed to the blockage of medium and strong acid sites as well as the high Ni crystallinity.

Attapulgite as a cost-effective catalyst for low-energy consumption amine-based CO2 capture

The extensive heat duty of solvent regeneration has significantly hampered the industrial application of amine-based CO2 capture technology. Catalytic CO2 desorption with solid acid catalysts has recently emerged as a promising and competitive strategy for improving solvent regeneration performance. The reported catalysts have exhibited poor activities and are relatively expensive, which has slowed the large-scale implementation of this technique. In this work, three low-cost and environmentally friendly clay catalysts, namely, attapulgite (ATP), activated bentonite (K-10), and sepiolite (SEP), were used to catalyze the CO2 desorption in the CO2-loaded 5 M monoethanolamine (MEA) solution at 88 °C. The findings revealed that ATP, K-10, and SEP catalysts accelerated CO2 desorption, and the ATP demonstrated better catalytic activity in the early stage of the desorption process. In particular, the introduction of ATP significantly increased the CO2 desorption rate by 54–57% and reduced the relative heat duty by about 32%, compared to the blank test. The superior catalytic performance of ATP might be attributed to its favorable surface area and medium acid sites. The catalytic effect of ATP was confirmed using FT-IR and Raman techniques, and a potential catalytic desorption mechanism was proposed. Furthermore, the stability of activated ATP was demonstrated using 15 CO2 absorption-desorption cycles. These results demonstrated that ATP significantly contributes to the advancement of the amine-based CO2 capture process and has significant potential for large-scale deployment. This work may pave the avenue for developing abundant and inexpensive clay-based solid acid catalysts for low-energy consuming CO2 capture technology.

Alumina catalyst with high density of medium acid sites for N-arylpyrrolidines synthesis

The N-alkylation of amines with tetrahydrofuran (THF) to synthesize pyrrolidine compounds is an economic and environmental benign route. However, current catalytic system often suffered from harsh reaction condition and hazardous reagents. In this work, we developed an alumina nanosheet catalyst with a large amount of medium acid sites for synthesis of N-arylpyrrolidines. Nearly 100% target product yield could be obtained at 190 °C, and this catalyst could be reused at least 5 times without obvious deactivation. NH3-temperature programmed desorption (TPD) and pyridine-infrared (py-IR) revealed that the catalytic performance is positively correlated with the content of medium acid sites. In-situ IR spectrum proved that the reaction follows the carbocation mechanism. This work affords a competitive system for green sustainable chemistry.

Dispersion and Stabilization of Supported Layered Double Hydroxide-Based Nanocomposites on V-Based Catalysts for Nonoxidative Dehydrogenation of Isobutane to Isobutene

Nonoxidative dehydrogenation of isobutane is one of the sustainable strategies for producing high value added isobutene. As alternatives for the commercial Pt- and Cr-based dehydrogenation catalysts, supported V-based catalysts are worthy of study. In this work, a series of VOx/mMgAlO-R catalysts (m = 10, 15, 20, 25 and 30) were designed and prepared by loading VOx on mMgAlO composite oxide supports derived from mesoporous Al2O3-supported layered double hydroxide (LDH) nanocomposites. The calcined and reduced catalysts were characterized by X-ray diffraction (XRD), Raman spectra, Ultraviolet-visible diffuse reflectance (UV-Vis) spectra, NH3 temperature-programmed desorption (NH3-TPD), Temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TG) and low temperature N2 adsorption–desorption isotherms. The as-synthesized VOx/20MgAlO-R with appropriate Mg addition exhibits superior activity (43–56% conversion and 77–81% selectivity), excellent stability and coking-resistance for the isobutane dehydrogenation. The structure–performance relationship reveals that the formation of VOx species confined in the reconstructed LDH interlayer and porous MgO facilitates dispersing and stabilizing the VOx species. The low polymerization degree and higher proportion of V4+ ion for VOx species, strong acidity of medium acid sites and low concentration of strong acid sites are responsible for the excellent anti-coking and catalytic performance. The strong VOx–support interaction is beneficial for enhancing the stability of the catalysts.

Supported molybdenum oxides for the aldol condensation reaction of acetaldehyde

The (retro-)aldol condensation reaction is an important chemical transformation in the upgrading of biomass-derived compounds into fuels and valuable specialty chemicals. In this study, we found that supported molybdenum oxide (MoOx) catalysts were active and selective for the aldol condensation of acetaldehyde to crotonaldehyde under steady-state reactor conditions. Through a combination of transmission electron microscopy (TEM), ultraviolet–visible (UV–VIS) diffuse reflectance spectroscopy, Fourier transform infrared (FTIR) spectroscopy of adsorbed pyridine, and steady-state reactor testing, we determined that highly dispersed MoOx has a strong interaction with a γ-Al2O3 support resulting in optimal catalyst performance at low weight loadings. In contrast, MoOx particles supported on SiO2 have a weaker interaction with the support, resulting in a monotonic relationship between Mo loading and aldol condensation activity. The Lewis acid site density and strength are important parameters for predicting aldol condensation activity across all samples. The concentration of weak acid sites had a poor correlation with aldol condensation activity, most likely because these sites are too weak to activate acetaldehyde for the reaction. Medium and strong acid sites both had good correlations to aldol condensation activity. Results from X-ray absorption near edge structure (XANES) and acetaldehyde temperature programmed desorption (TPD) indicated that partially reduced MoOx was more active for aldol condensation, but pretreatment in reducing or oxidizing environments had no significant effect on steady-state catalytic activity. Characterization of spent catalyst samples through temperature programmed oxidation (TPO) and thermogravimetric analysis (TGA) revealed that catalysts with high densities of strong acid sites tended to form more carbonaceous deposits on the surface over the course of the reaction.

Hierarchical zeolites as efficient catalysts for dehydration of substituted indanols

Hierarchical zeolites of different topology and morphology (Al-, Ga- and B-silicate zeolites of MOR, BEA, MFI and MTW structural types with the morphology of aggregated nanosheets, nanorods, or close to spherical nanoparticles) were evaluated in the dehydration of a homologous series of substituted 4-bromo-2-alkyl-2,3-dihydro-1H-indan-1-ols. The optimal strength of the catalyst's acid sites providing both high indanols conversion and selectivity towards indenes (up to 99%) was found for each of the studied substrates. Stronger acid sites than optimal ones resulted in the formation of by-products of self-alkylation of the initial indanols by the formed indenes or alkylation of benzene used as a solvent. Weak acidity of the catalysts led to low substrate conversion and the presence of the corresponding ether. A correlation between the catalyst acidity and the yield of the desired product was found. Medium strength acid sites were discovered to be preferable for the selective dehydration of substituted 4-bromo-2,3-dihydro-1H-indan-1-ol.

Higher alcohol production from ethanol over occluded [Mg4(OH)4]4+ clusters in MgO/KNaX

Conversion of ethanol to higher alcohols was studied over MgO/KNaX, prepared by ion exchange with Mg(OAc)2, followed by KOH washing. The catalysts were characterized by XRF, XRD, SEM, BET, 27Al MAS NMR, EXAFS, NH3- and CO2-TPD. All catalysts showing MgO nanopetals and aggregates on the external surface, contained occluded [Mg4(OH)4]4+ clusters in the zeolite cavities, providing medium basic (Mb) and acid (Ma) sites. Ethanol conversion and higher alcohols selectivity (up to 78%) increased with Mb/Ma ratio due to the increase in both MgO (4–6 wt%) and K (14.7–17.3 wt%) loadings. Decreasing occluded [Mg4(OH)4]4+ clusters and/or increasing MgO aggregates led to the lower conversion and yields of higher alcohols. The essential role of the occluded [Mg4(OH)4]4+ clusters in producing higher alcohols was verified by the reactions using various control catalysts. The MgO/KNaX showed high stability even after steaming at 380 °C, as well as being regenerated by calcination (450 °C in air).

Surface properties of nano mesoporous lanthanum oxide synthesized by sol-gel method

This study investigates surface properties like surface acidity/basicity, surface reducibility, surface area, and porosity of nano mesoporous lanthanum oxide (La2O3) synthesized by the sol–gel method. X-ray diffraction (XRD) analysis confirms the formation of nanocrystalline hexagonal La2O3 at 800 °C. Fourier transform infrared spectra (FT-IR) shows the peaks corresponding to surface carbonate and surface water on La2O3. Morphology of La2O3 was studied by transmission electron microscopy (TEM). The surface acidity of La2O3 was measured by temperature-programmed desorption (TPD-NH3) analysis and surface basicity by TPD-CO2. TPD profiles show the presence of medium and strong acidic and basic sites on the surface of La2O3. La2O3 has more surface acidity compared to surface basicity. Temperature programmed reduction (H2-TPR) analysis indicates good stability of La2O3 towards reduction. The values of H2 consumption indicate a greater amount of reducible species on the surface. Surface area and porosity measurements were done using Braunauer-Emmet-Teller (BET) analysis. The pore size distribution curve is also determined from the Barrett-Joyner-Halenda (BJH) adsorption/ desorption isotherm. La2O3 is found to be mesoporous with pore diameter of 20.6 nm and has wide pore size distribution between 5 and 50 nm. Typical type IV isotherm with H3 type hysteresis loops at a high P/P0 value is observed for La2O3.

Enhancing Methane Conversion by Modification of Zn States in Co-Reaction of MTA

Limited by harsh reaction conditions, the activation and utilization of methane were regarded as holy grail reaction. Co-reaction with methanol, successfully realizing mild conversion below 450 °C, provides practical strategies for methane conversion on metal-loaded ZSM-5 zeolites, especially for highly efficient Zn loaded ones. However, Zn species, regarded as active acid sites on the zeolite, have not been sufficiently studied. In this paper, Zn-loaded ZSM-5 zeolite was prepared, and Zn was modified by capacity, loading strategy, and treating atmosphere. Apparent methane conversion achieves 15.3% for 1.0Zn/Z-H2 (16.8% as calculated net conversion) with a significantly reduced loading of 1.0 wt.% against deactivation, which is among the best within related zeolite materials. Besides, compared to the MTA reaction, the addition of methane promotes the high-valued aromatic production from 49.4% to 54.8%, and inhibits the C10+ production from 7.8% to 3.6%. Notably, Zn2+ is found to be another active site different from the reported ZnOH+. Medium strong acid sites are proved to be beneficial for methane activation. This work provides suggestions for the modification of the Zn active site, in order to prepare highly efficient catalysts for methane activation and BTX production in co-reaction with methanol.

Hydrodeoxygenation of fatty acid over La-modified HZSM5 for premium quality renewable diesel production

In the present study, a series of La-HZSM5 catalysts were prepared through wet impregnation and investigated for the production of premium quality diesel fuel via hydrodeoxygenation (HDO) of oleic acid (OA). Several parameters were evaluated, such as reaction temperature (360–400 °C), H2 pressure (1–5 MPa), La concentration (5–20%), catalyst loading (1–7 wt%), and reaction time (1–4 h). At a reaction of 400 °C for 2 h under 5 MPa H2 pressure, the La(10)HZSM5(90) catalyst showed 97% hydrocarbon yield with 92% of diesel selectivity. The superior HDO activity by La(10)HZSM5(90) in producing high quality diesel is due to a sufficient number of weak + medium acid sites and its mesoporous structure. Vacuum distillation was complementary for producing high quality diesel fractions consisting of C17 fractions. The reusability of the La(10)HZSM5(90) catalyst was also demonstrated, though the catalyst still had coking activity.

The Role of Medium Acid Sites Tuned by Ce Adding in Moderate-Temperature NH3-SCR

The Role of Medium Acid Sites Tuned by Ce Adding in Moderate-Temperature NH3-SCR

In Situ Hydrodeoxygenation of Lignin-Derived Phenols With Synergistic Effect Between the Bimetal and Nb2O5 Support

Nb2O5-supported bimetallic catalysts were prepared by the impregnation method applied for the in situ hydrogenation of guaiacol. Guaiacol can be effectively transformed into cyclohexanol over different bimetallic catalysts using alcohol as the hydrogen donor. Meanwhile, the effects of different hydrogen donors such as isopropanol, sec-pentanol, and ethylene glycol on in situ hydrogenation of guaiacol were investigated in detail, and the results showed that isopropanol is the best hydrogen supply solvent. Then, the dependence of Ni–Mn/Nb2O5 properties on metal loading, reaction time, reaction temperature, and reaction pressure was studied for the in situ hydrogenation of guaiacol by using isopropanol as the hydrogen donor. Guaiacol can be completely converted, and the yield of cyclohexanol reached 71.8% over Ni–Mn/Nb2O5 with isopropanol as the hydrogen donor at 200°C for 5 h. The structures and characteristics of better catalytic properties of the Ni–Mn/Nb2O5 catalyst were determined by BET, NH3-TPD, XRD, XPS, SEM, and TEM, and the results indicated the particle size of the metal was small (approximately 10 nm) and the metal particles are finely dispersed in the whole support. Therefore, a large number of medium acid sites were generated on the 10Ni-10Mn/Nb2O5 with a large specific surface area, which could increase the interface between the metal and the support and may be beneficial to the hydrodeoxygenation of guaiacol.

Effect of cu content on Ce‐doping CuO/ZrO2 catalysts for low‐temperature hydrogenation of dimethyl oxalate to ethanol

AbstractIn order to solve the Cu sintering in dimethyl oxalate (DMO) hydrogenation to ethanol, the reaction temperature should be kept at a low value, however, which is unable to show excellent catalytic performance. Herein, the reaction temperature decreased significantly from above 260 to 220°C over Ce‐doping Cu/ZrO2 catalyst, which retained high catalytic efficiency with little influence on the catalyst structure after 96 h on stream. Besides, the structure and performance were changed greatly by adjusting the Cu content. For example, the ethanol selectivity can reach 96.1% based on the 100% conversion of DMO over Ce‐doping Cu/ZrO2 catalysts with 40% content of Cu. Based on systematic characterization, the catalyst possessed higher exposed Cu surface area along with the proper amount of medium acidic sites, avoiding the strong ones that could bring about a series of by‐products. As a result, the formed synergistic effects between Cu sites and medium acidic sites provided the active sites with high efficiency for ethanol synthesis.

Facile and Efficient Synthesis of Primary Amines via Reductive Amination over a Ni/Al2O3 Catalyst

The production of primary amines with high efficiency and selectivity employing appropriate catalysts is of central importance yet challenging. Here, we reported Ni nanoparticles supported on γ-Al2O3 as a highly active heterogeneous catalyst for the synthesis of primary amines through reductive amination in the presence of ammonia and hydrogen. The resulting Ni/Al2O3 catalyst exhibited outstanding performance for the selective reductive amination of carbonyl compounds (aldehydes and ketones) involving aromatic, branched, and purely aliphatic chemicals converted into their corresponding primary amines with excellent yields (87∼99%) under mild reaction conditions (100 °C and 2 MPa), which was attributed to the synergistic effect of strong metal–support interaction combined with medium acidic sites. In particular, the yield of primary amines for earth-abundant Ni catalysts was superior to that for precious metal catalysts (Ru, Rh, and Pd), suggesting that the Ni/Al2O3 catalyst could be a promising candidate for the industrial production of amines.